The invention relates to an optical glass fibre having a synthetic resin cladding, comprising a glass fibre and an enveloping layer of a synthetic rubber having a refractive index which is higher than the refractive index of the outermost layer of the glass fibre, the synthetic rubber being formed from a curable synthetic resin composition comprising a siloxane copolymer which has monomeric units comprising dimethyl siloxane and at least one siloxane selected from the group formed by methyl phenyl siloxane and diphenyl siloxane, which siloxane copolymer comprises at least two acrylate groups per molecule.
Such optical glass fibres are used for transmitting light signals for telecommunication purposes.
Such an optical glass fibre is described in Netherlands Patent Application No. 8400727. The siloxane copolymer employed has a number average molecular weight between 1,000 and 25,000.
The curable synthetic resin composition may further comprise one or several monomeric acrylate compounds as well as an initiator which can be activated by light.
Immediately after the glass fibre has been formed the curable synthetic resin composition is applied onto the glass fibre and subsequently cured by means of UV radiation or radiation by electrons. The curing time is shorter than 5 seconds, preferably shorter than 0.5 second.
The synthetic rubber thus obtained has a refractive index which is larger than 1.46, a modulus of elasticity between 0.1 and 10 MPa and a glass transition temperature of -50.degree. C. or lower. However, this synthetic rubber has a low resistance to wear and a low mechanical strength which complicates transport and storage of the optical glass fibre and the working of said fibre into a cable.
It is possible to increase the mechanical strength by adding an inorganic filler to the synthetic rubber, yet this has the disadvantage that the filling particles may damage the optical glass fibre. In general, the soft layer of synthetic rubber is protected by a second, harder synthetic resin cladding.